Nyarko, Benjamin Kofi
Figure 1.4 Total discharges of sections within the study Area.
TOTAL DISCHARGE OF SECTIONS OF THE STUDY AREA
Discharge (m3/sec)
1000.0
0
1:1178872
4.2 The modified relational model in risk zone determination
Stages three and four of the modified relational flow models was followed to generate the flood
risk map. These are:
e The application of the rule using arithmetic overlay operation, addition and division to
help integrate the hydrological model into the geographic information system model.
e The generation of flood risk zone maps for Accra and its environs.
4.3 Mode of model integration
Among the various methods of determining runoff, it has been noted that the combination of a
physical, deterministic or hybrid model with a digital elevation model within the geographic
information system model offers an alternative to these models to give a spatial view of a
phenomenon as its end result. Therefore, to achieve this an attempt was made to use an
arithmetic overlay method to combine DEM and discharge map within the geography
information system model to determine flood risk areas. Operators such as addition and division
were used in the combination procedure. The essence of using the arithmetic overlay method is
that possibilities exist for the derivation and examination of spatial patterns caused by
interactions of one map with the other. Secondly, the rule also provided the possibility of
restricting areas on the output map according to a binary map that act as a mask.
The arithmetic overlay method used involves two main stages:
1. The first stage involves the determination of runoff within various segments over the
landscape.
XY, (2)
2. The second stage is estimation of values that can be used to infer potential areas that are
likely to be in flood with any storm event.
XY, (3)
x FRA
Where:
X(m) is the Digital Elevation Model
Y(m°/Sec) represent total discharge
1044 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000.
